Dialkyl dithiophosphates and lubricants containing them



This application is a continuation-in-part of Frank A. Stuart, U.S. application Serial No. 224,550, filed September 18, 1962, now abandoned.

This invention relates to novel dialltyl dithiophosphates. More particularly the invention concerns new zinc dialkyl dithiophosphates having improved thermal stability.

Dialkyl dithiophosphates are useful for a variety of purposes known to the art. The zinc dialkyl dithiophosphates in particular are employed as oxidation and corrosion inhibitors in petroleum hydrocarbon compositions, such as lubricating oils, fuels, greases and asphalts.

In lubricating compositions, high temperature performance has become more and more critical. Modern, more efiicient engines are designed to operate at increasingly higher temperatures, and it is necessary that the lubricating oils and greases withstand these high temperatures with as little deterioration as possible.

In accordance with the present invention, it has been found that zinc dialkyl dithiophosphates of improved thermal stability are provided in the zinc salts of mixed dialkyl dithiophosphoric acids in which the alkyl groups are derived from a mixture of at least two different branched chain primary alcohols, one of said alcohols being isobutyl alcohol and the other of said alcohols containing at least 6 carbon atoms.

The mixed dialkyl dithiophosphoric acids are conveniently obtained by reacting phosphorus pentasulfide with the mixture of the at least two different alcohols. However, other conventional methods of preparing mixed dialkyl diesters of dithiophosphoric acids known to the art may be employed. The diesters are generally characterized as having the structural formula:

wherein radicals R and R are the same or different branched chain primary alcohols derived from mixtures of at least two different branched chain primary alcohols, one of said alcohols being isobutyl alcohol and the other of said alcohols containing at least 6 carbon atoms.

In the mixtures of branched chain primary alcohols, the mole ratio of the isobutyl alcohol to those alcohols having at least 6 carbon atoms should be within the range of about 0.521 to about 8:1 and preferably from about 1:1 to about :1.

Suitable branched chain primary alcohols containing at least 6 carbon atoms include Z-ethylbutyl alcohol, otherwise known as 2-ethyl-l-butanol, 2-methyl-1-pentanol, 3-methyl-l-pentanol, 4-methyl-1-pentanol, isoheptyl alcohol, Z-ethyl-I-pentanol, 3-ethyl-l-hexanol, isooctyl alcohol, 2-propyl-1-pentanol, decyl alcohol, dodecyl alcohol, octadecyl alcohol, etc. For present purposes, the 6 carbon atom alcohols are preferred, although alcohols having as high as 10 carbon atoms or more are quite satisfactory.

The zinc salts of the mixed dialkyl dithiophosphoric acids are also conveniently prepared in accordance with conventional procedures for this type of reaction. The mixed dialkyl dithiophosphoric acids prepared by reacting the phosphorus sulfide, such as P 3 with the mixed alcohols is readily neutralized with a basic compound of zinc, such as zinc oxide.

atent O The following example is offered in further illustration of the compounds of the invention. Unless otherwise specified, the proportions given are on a weight basis.

Example A mixture of 261 g. of isobutanol (3.52 moles) and g. of 2-ethylbutanol (0.88 mole) is introduced to a reaction vessel. To this mixture is added 222 g. of phos phorus pentasulfide (P S This is a 10% weight excess of the alcohols over the theoretical required to form the dialkyl dithiophosphoric acid. The mixture is heated at about 170 R for 2 hours. The weight loss of H 5 is about 35 g., and the unreacted P S is about 4.4 g.

The acid mixture obtained is allowed to stand cold and quiescent overnight. Following this, 450 g. of the acid mixture is added to 220 g. of mineral lubricating oil having a viscosity of about 300 SSU at F. To the mineral lubricating oil-acid mixture is then added 82.3 g. of zinc oxide which is of theoretical based on 91% purity of the acids. The temperature is maintained at about F. for about 2 hours. After this, another 3 g. of zinc oxide is added and the heating continued for 4 hours. An additional amount of 3 g. of zinc oxide is added and the mixture heated for about 6 hours, at which time the pH is 6.43.

The mixture of neutralized dialkyl dithiophosphoric acid in oil is air-blown after the product has stood overnight. The air blowing is continued for about 1 /2 hours and the product filtered through a clay filter, and has the following analysis:

P=7.2% Zn=8.44%

The above analysis corresponds to the theoretical for the zinc isobutyl, 2ethylbutyl dithiophosphate.

The product prepared in the above example is tested in a typical oxidation test. A sample of the oil containing the zinc dialkyl dithiophosphate is heated while air is passed through the oil. It is further found that the product of this example may be heated at temperatures as high as 490 F. before decomposition, whereas a comparable Zinc salt of the reaction product of secondary butyl alcohol, methylisobutyl carbinol and P 5 decomposes at about 410 to 415 F. It is also found that conventional copper strip tests give a small weight loss and good appearance with very slight stain.

Comparable test results are obtained as follows, showing the improved thermal stability of the zinc dialkyl dithiophosphates of the present invention. These tests are obtained in the conventional L38 engine test procedure. Oil A is a mineral lubricating oil of SAE 30 weight containing the copolymer of dodecyl methacrylate, octadecyl methacrylate, polyethylene glycol methacrylate (1800 mole weight) and glycidyl methacrylate-dimethylaminopropylene diamine reaction product in about 2.5% by weight. Oil B is an SAE 40 mineral lubricating oil containing about 3% by weight of calcium petroleum sulfonate. For obvious reasons, the thiophosphate in each comparison is used in the same and in conventional amounts; that is, Oil A contained 10 millimoles per kilogram and Oil B 15 millimoles per kilogram. As a result of the test, it is found that the viscosity loss of Oil A containing the zinc salt of the invention as measured in SSU at 100 F. amounts to 1.0%. By comparison, the viscosity change of Oil A containing zinc isobutyl mixed primary amyl dithiophosphate, a commercial additive as described by US. Patent No. 3,000,822, results in a viscosity loss of 1.8%, or about twice as much. In Oil B there is no viscosity loss with the present additive, whereas with the aforementioned comparable commercial additive the viscosity loss is 8.6%, or more than eight times as much.

c.) The total of the varnish ratings, sludge ratings and overall engine ratings are the same in each comparison.

In other severe tests run with the compositions of the present invention in comparison with the commercial zinc dialkyl dithiophosphate, it is found, for example, that a 120 hour Supplement 1 Caterpillar test gives substantial improvement in the ring groove deposits and ring land deposits.

In addition to the mineral lubricating oil composition described above, the present invention also concerns oils of lubricating viscosity in general. Such oils include synthetic oils of the dicarboxylic acid ester type, such as di- Z-ethylhexyl sebacate. The synthetic oils include tricresyl phosphate. Also included is the methyl polyisopropylene ether having a molecular weight of about 1000. These and other known synthetic oils are benefited by the addition of the improved new zinc dialkyl dithiophosphate.

In the finished lubricating oils, fuels, greases, etc. in accordance with the invention, the zinc dialkyl dithiophosphate is employed in amounts sufiicient to inhibit oxidation and corrosion. Usually amounts of 0.25% to about 20% by weight, preferably from about 1.5% to by weight are used. However, the excellent solubility characteristics of the mixed zinc dialkyl dithiophosphates render them suitable for the preparation of lubricating oil concentrates containing higher percentages of the dithiophosphate up to about 75% by weight in base lubricating oil.

Lubricant compositions within the scope of the present invention may also contain still other additives of conventional types such as detergents, pour point depressant viscosity index improvers, oiliness and extreme pressure agents, antioxidants, dyes, blooming agents and the like. Illustrative lubricating compositions of the aforementioned type may contain from about 0.1 to about 10% by weight of metallic detergents, calcium alkyl phenate having mixed alkyl groups of 12 to 15 carbon atoms or calcium petroleum sulfonates of the oil soluble mahogany type. Likewise the lubricant compositions may contain from about 0.1 to 10% by Weight of ashless detergents such as the polymeric type mentioned above or the alkenyl succinimide of tetracthylene pentamine having from about 30 to 300 carbon atoms in the alkenyl group.

While the character of this invention has been described in detail with numerous examples, this has been done by way of illustration only and without limitation of the invention. It will be apparent to those skilled in the art that numerous modifications and variations of the illustrative examples may be made in the practice of the invention within the scope of the following claims.

I claim:

11. Zinc salts of mixed dialkyl dithiophosphoric acids in which the alkyl groups are derived from a mixture of isobutanol and a branched chain primary hexyl alcohol, the mole ratio of isobutanol to hexyl alcohol being within the range of 0.1:1 to 8:1.

2. Zinc salts of mixed dialkyl dithiophosphoric acids in which the alkyl groups are derived from a mixture of isobutanol and 2-ethylbutanol, the mol ratio of isobutanol to 2-ethylbutanol being Within the range of 1:1 to 5:1.

3. Zinc salts of mixed dialkyl dithiophosphoric acids in which the alkyl groups are derived from a mixture of isobutanol and methyl-l-pentanol, the mole ratio of isobutanol to methyl-l-pentanol being within the range of 1:1 to 5:1.

4. A lubricant composition consisting essentially of a major proportion of an oil of lubricating viscosity and a minor proportion sufficient to inhibit oxidation and corrosion of the zinc salt of claim 1.

5. A lubricant composition consisting essentially of a major proportion of an oil of lubricating viscosity and a minor proportion sufiicient to inhibit oxidation and corrosion of the zinc salt of claim 3. 

1. ZINC SALTS OF MIXED DIALKYL DITHIOPHOSPHORIC ACIDS IN WHICH THE ALKYL GROUPS ARE DERIVED FROM A MIXTURE OF ISOBUTANOL AND A BRANCHED CHAIN PRIMARY HEXYL ALCHOL, THE MOLE RATIO OF ISOBUTANOL TO HEXY ALCOHOL BEING WITHIN THE RANGE OF 0.1: TO 8:1.
 4. A LUBRICANT COMPOSITION CONSISTING ESSENTIALLY OF A MAJOR PROPORTION OF AN OIL OF LUBRICATING VISCOSITY AND A MINOR PROPORTION SUFFICIENT TO INHIBIT OXIDATION AND CORROSION OF THE ZINC SALT OF CLAIM
 1. 